Wyo VB Lecture Notes

Objective #1: Use While loops.

A While loop can be used to execute a block of code many times over and over. The loop below causes a message box to appear 4 times.

Dim num As Integer = 0

While (num <= 3)
   MessageBox.Show("Hello World")
   num = num + 1
End While

In the While loop above     (num <= 3)      is a control expression just like the control expressions in If statements that you studied earlier in this course.

The statements

MessageBox.Show("Hello World")
num = num + 1

are considered the body of this loop since they are indented and placed between the While and the End While.
The loop only iterates if the control expression is True. If the control expression is True then VB executes the indented statements in the body of the loop. The body of the loop executes over and over again until the control expression becomes False. Each time a loop executes, it is called an iteration of the loop.
When the control expression evaluates to False, the code inside the loop is not executed and the loop terminates. If the control expression never becomes false, the loop would execute forever. A loop that executes forever is called an infinite loop.
For good style, you should indent the statements inside the body of the loop and you should place parentheses around the control expression for readability. You should always surround your loops with blank lines in your code. This makes your program easier to read and debug. That is, above all While and For loops you should include a blank line and below all End While and Next statements, you should include a blank line. Blank lines used to make code easier to read are called white space. Tabs and blank spaces that are also used for the same purpose of making white space.

Objective #2: Trace While loops.

Here is an example of a While loop that adds up the numbers from 1 to 5

Dim num As Integer = 1
Dim sum As Integer = 0

While (num <= 5)
sum += num
num += 1
End While

The variable num will end up having the values: 1, 2, 3, 4, 5, 6
The variable sum will end up having the values: 1, 3, 6, 10, 15
However the loop can be modified to add up the values from 1 to 10 by changing the 5 in the control expression to 10

Dim num As Integer = 1
Dim sum As Integer = 0

While (num <= 10)
   sum += num
   num += 1
End While

The loop can be modified to add the values from 5 to 10 by changing the initial value of num

Dim num As Integer = 5
Dim sum As Integer = 0

While (num <= 10)
   sum += num
   num += 1
End While

or the loop could be used to add up the even numbers between 2 and 10 by setting the initial value of num to 2 and changing one of the body statements

Dim num As Integer = 2
Dim sum As Integer = 0

While (num <= 10)
   sum += num
   num += 2
End While
Here is an example of a While loop that determines how many years it would take to make $100 turn into $150 at a 5% interest rate compounded yearly.

balance = 100
years = 0

While (balance < 150)
balance = balance + balance * 0.05
years += 1
End While

Objective #3: Use sentinel values to control While loops.

A sentinel value is often used to determine when an indefinite While loop should terminate. The user is prompted to input a value such as -1 to signal to the loop that it should terminate.
For example, if you ask the user to input his test scores but your program does not necessarily know how many tests he has taken you could use a While loop and simply tell the user to enter the number -1 to end his list of input values. Of course, we know that -1 will not be confused with his actual exam scores since it is impossible to earn a -1 on an exam. In this case, -1 is a sentinel value that warns the loop that the inputs have ended just like Paul Revere was a sentinel that warned the Americans that the British were coming.
Here is an example of a While loop that uses a sentinel value to compute the sum of numbers inputed by the user until the user inputs -1. In this example, the value -1 is used as a sentinel value.
Dim num As Integer = 0
Dim sum As Integer = 0

While (num <> -1)
    sum += num
    num = InputBox("Enter an integer (-1 to quit)")
End While
You can use an Exit While statement to exit or "break out of" a While loop. In this loop, the loop immediately stops iterating when the user inputs the sentinel value of -1 because the Exit While statement makes the loop immediately end.

Dim num As Integer = 0
Dim sum As Integer = 0

While (num <> -1)
   num = InputBox("Enter an integer (-1 to quit)")

   If (num = -1)
      Exit While
   End If

   sum += num
End While
You must be careful not to add your sentinel value into your running total. The following While loop contains a logic error

Dim num As Integer = 0
Dim sum As Integer = 0

While (num <> -1)
   num = InputBox("Enter an integer (-1 to quit)")
   sum += num
End While

In this case, the value -1 is added into the running total even though that is not the programmer's intent. So it could be fixed by using the Exit While statement that is used in an earlier example or by adding 1 to sum after the loop is finished

Dim num As Integer = 0
Dim sum As Integer = 0

While (num <> -1)
   num = InputBox("Enter an integer (-1 to quit)")
   sum += num
End While

sum = sum + 1   ' to make up for the extra input of -1
Here is an example of a While loop that counts how many teenagers there are in a group of people. Again, -1 is used as a sentinel value. The value -1 is chosen since it is not a realistic age. You would not use the value 0 since an infant who is 6 months old would be considered 0.

Dim total As Integer = 0
Dim age As Integer = 0

While (age <> -1)
   age = InputBox("What is your age?")

   If (age >= 13 And age <= 19) Then
      total += 1
   End If

End While

Objective #4: Use flag variables to control While loops.

A flag variable is often used to store the value 1 (meaning True) or 0 (meaning False). The programmer uses the flag variable as a way of controlling some process in an algorithm. Flag variables are often used in While loops to send a signal to the control expression. They are not used in For loops since a For loop executes a predetemined, definite number of times.

For example, in the code segment below gameIsOver is used as a flag variable. It sends a signal to the control expression of the loop if the user has scored enough points to end the game and exit the loop. gameIsOver is initially set to the value 0 which indicates False (the game is NOT over). You should name flag variables appropriately. The variable should be named in a way that it's name indicates a True condition. (For example, in the segment below, you should not use a flag variable with the name, gameIsNotOver, since it would probably make the logic of the algorithm harder for most fellow programmers to follow.)

Dim gameIsOver As Integer = 0

While (gameIsOver = 0)
    userPointsScored = userPointsScored + lastRoundScore

    If (userPointsScored > 1000) Then
        gameIsOver = 1
    End If

End While

Often programmers use variables with the Boolean data type as flag variables. A Boolean variable can be set to either True or False. Here is an example:

Dim gameIsOver As Boolean = False

While (gameIsOver = False)
    userPointsScored = userPointsScored + lastRoundScore

    If (userPointsScored > 1000) Then
        gameIsOver = True
    End If

End While

By using a Boolean variable you can simplify the loops control expression to this:

While (Not(gameIsOver))

Here's another example of a While loop that uses a flag variable. In this case, the flag variable has the Boolean data type.

Dim outOfBounds As Boolean = False

While (outOfBounds = False)
   picPlayer.Left = picPlayer.Left + 10

   If (picPlayer.Left > 300) Then
      outOfBounds = True
   End If

End While

Objective #5: Know when to use While loops rather than For loops.

A While loop should be used instead of a For loop when you do not know the exact number of iterations that the loop must make. If there is a situation in which some process must occur over and over again within a loop but you do not know when exactly it will end then you should use a While loop.
Typically a While loop is used when a user provides input and the numbers or data that the user is inputs is unpredictable. For example, if a customer is making a number of purchases and the programmer does not know exactly how many items will be purchased beforehand. Or, if a user is moving an object across the screen but the programmer does not know exactly where the object will stop.

A While loop is called an indefinite loop (or indeterminate loop) because the programmer does not know beforehand exactly how many times it will iterate. Remember that a For loop is a definite loop (or determinate loop) because a programmer can count how many times it will iterate based on its initial value, limit value, and step value.
Technically though any For loop could be rewritten as a While loop and any While loop could be rewritten as a For loop. Consider this For loop

For J = 1 To 3
   sum += J
Next

It could be rewritten as

While (J <= 3)
   sum += J
   J += 1
End While

Even this While loop that uses a sentinel value and a flag variable

While (Not (exitLoop))
   num = InputBox("Enter a number: ")

   If (num = -1) Then
      exitLoop = True
   Else
      sum += num
   End If

End While

could be rewritten as a For loop

For J = 1 to 2
   num = InputBox("Enter a number: ")

   If (num = -1) Then
      J = 3
   Else
      sum += num
      J -= 1
   End If

Next

though this is horrible style since the control variable J is not being allowed to step to 2 as is meant with a For loop.

The bottom line is that if the user is inputing numbers and the programmer is not sure how many numbers will be inputed, the programmer should use an indefinite While loop along with a sentinel value and/or a flag variable.

If the programmer knows exactly how many loop iterations he/she is expecting to occur, he/she should use a definite For loop.